Such contactlessly working measuring devices are applied in a number of branches of industry, e.g. in the processing industry, in the various industrial applications of chemistry and in the foods industry.
FIG. 1 shows such an arrangement for fill level measurement, such as it is today typically applied. In such case, the fill substance 1 is located in a container 3, on which the fill-level measuring device 5 working with microwaves according to the travel time principle is arranged. The fill-level measuring device 5 includes an antenna 7, via which microwaves generated by a measuring device electronics 9 are sent and their echoes reflected on the surface of the fill substance are received back, after a travel time dependent on the fill level, and fed to the measuring device electronics 9. The measuring device electronics 9 ascertains the associated travel time and calculates therefrom, on the basis of the propagation velocity of the microwaves and the installation height of the measuring device 5, the fill level.
There are a number of applications in which the fill-level measuring device, or its antenna, cannot be arranged directly over the fill substance. An example of this are applications, in which, above the fill substance, very demanding environmental conditions reign, which the antenna, or the measuring device, especially its electronic components, cannot withstand. Thus, for example, in the case of containers filled with very hot fill substances, e.g. molten metals, there can occur above the fill substance enormously high temperatures, which would, within a very short time, lastingly damage, or even completely destroy, the measuring device, or its antenna. Further examples include applications with very large vapor, or dust, development. Both vapor and dust deposit on the antenna and lead to significant degrading of the transmitting and/or receiving characteristics of the measuring device.
Further examples include applications, in which fill levels at two or more measuring locations spatially separated from one another are to be measured with a single fill-level measuring device.